Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1-2 illustrate a conventional vacuum IG unit (vacuum IG unit or VIG unit). Vacuum IG unit 1 includes two spaced apart glass substrates 2 and 3, which enclose an evacuated or low pressure space 6 therebetween. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess 11 retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution therebetween. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated to a temperature of approximately 500° C., at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500° C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500° C. for one hour. Inventor Collins of the '395 patent states in “Thermal Outgassing of Vacuum Glazing,” by Lenzen, Turner and Collins, that “the edge seal process is currently quite slow: typically the temperature of the sample is increased at 200° C. per hour, and held for one hour at a constant value ranging from 430° C. and 530° C. depending on the solder glass composition.” After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
Unfortunately, the aforesaid high temperatures and long heating times of the entire assembly utilized in the formulation of edge seal 4 are undesirable, especially when it is desired to use a heat strengthened or tempered glass substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates in certain instances.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (° F.). The different curves/lines are 400° F. (across the top of the FIG. 3 graph), 500° F., 600° F., 700° F., 800° F., 900° F., and 950° F. (the bottom curve of the FIG. 3 graph). A temperature of 900° F. is equivalent to approximately 482° C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900° F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900° F. or 482° C.). Such a significant loss (i.e., 80% loss) of temper strength is of course undesirable.
In FIGS. 3-4, it is noted that much better temper strength remains in a thermally tempered sheet when it is heated to a temperature of 800° F. (about 428° C.) for one hour as opposed to 900° F. for one hour. Such a glass sheet retains about 70% of its original temper strength after one hour at 800° F., which is significantly better than the less than 20% when at 900° F. for the same period of time.
Another advantage associated with not heating up the entire unit for too long is that lower temperature pillar materials may then be used. This may or may not be desirable in some instances.
Even when non-tempered glass substrates are used, the high temperatures applied to the entire VIG assembly may melt the glass or introduce stresses. These stresses may increase the likelihood of deformation of the glass and/or breakage.
Thus, it will be appreciated that there is a need in the art for a vacuum IG unit, and corresponding method of making the same, where a structurally sound hermetic edge seal may be provided between opposing glass sheets. There also exists a need in the art for a vacuum IG unit including tempered glass sheets, wherein the peripheral seal is formed such that the glass sheets retain more of their original temper strength than with a conventional vacuum IG manufacturing technique where the entire unit is heated in order to form a solder glass edge seal.
An aspect of certain example embodiments of this invention relates to applying localized heating to the periphery of a unit to form edge seals to reduce the heating of the non-peripheral areas of the unit and thereby reduce the chances of the substrates breaking.
An aspect of certain example embodiments relates to providing staged heating, localized heating, and staged cooling of a unit via a unitized oven, the localized heating being provided by a substantially linear focused infrared (IR) heat source comprising an array or matrix of linear heat sources.
Another aspect of certain example embodiments relates to providing a vacuum IG unit having a peripheral or edge seal formed so that at least certain portion(s) of thermally tempered glass substrates/sheets of the vacuum IG unit retain more of their original temper strength than if conventional edge seal forming techniques were used with the solder glass edge seal material.
Another aspect of certain example embodiments relates to providing a vacuum IG unit, and method of making the same, wherein at least a portion of the resulting thermally tempered glass substrate(s) retain(s) at least about 50% of original temper strength after formation of the edge seal (e.g., solder glass edge seal).
Another aspect of certain example embodiments relates to reducing the amount of post-tempering heating time necessary to form a peripheral/edge seal in a vacuum IG unit.
In certain example embodiments of this invention, an apparatus for forming an edge seal in a vacuum insulated glass (VIG) unit is provided. A plurality of infrared (IR) heating elements are controllable to emit IR radiation at a peak wavelength in the near infrared (NIR) and/or short wave infrared (SWIR) band(s). The IR heating elements are spaced apart from one another so as to have a 2-6″ center-to-center distance. The IR heating elements are vertically positioned 1-36″ (more preferably 2-10″) above an upper surface and/or below a lower surface of a VIG subassembly insertable therein. A controller is operable to adjust an amount of voltage supplied to the plurality of IR heating elements to vary the peak wavelength produced by the plurality of IR heating elements. Inner walls of the apparatus comprise a material having characteristics suitable for causing a reduced amount of IR radiation from the IR heating elements impinging thereon to be reflected, with the reflected IR radiation being reflected in a generally diffuse or undirected pattern. Insulation is provided around the inner walls.
In certain example embodiments of this invention, a method of making a vacuum insulated glass (VIG) unit comprising an edge seal is provided. A VIG subassembly is inserted into an apparatus including a plurality of infrared (IR) heating elements controllable to emit IR radiation at a peak wavelength in the near infrared (NIR) and/or short wave infrared (SWIR) band(s), with the plurality of IR heating elements being spaced apart from one another so as to have a 2-6″ center-to-center distance and being vertically positioned 2-10″ above an upper surface and/or below a lower surface of the VIG subassembly. Inner walls of the apparatus comprise a material having characteristics suitable for causing a reduced amount of IR radiation from the IR heating elements impinging thereon to be reflected, with the reflect IR radiation being reflected in a diffuse or undirected pattern. Insulation is provided around the inner walls. Frit material provided around the periphery of the VIG subassembly is heated via the plurality of IR heating elements in forming the edge seal, with the amount of voltage being supplied to the plurality of IR heating elements being adjustable to vary the peak wavelength produced by the plurality of IR heating elements so as to preferentially heat the frit material compared to glass substrates of the VIG subassembly.
In certain example embodiments of this invention, an apparatus for forming an edge seal in a vacuum insulated glass (VIG) unit is provided. A plurality of infrared (IR) heating elements are controllable to emit IR radiation at a peak wavelength in the near infrared (NIR) and/or short wave infrared (SWIR) band(s). A controller is operable to adjust an amount of voltage supplied to the plurality of IR heating elements to vary the peak wavelength produced by the plurality of IR heating elements. The controller is operable in first and second modes, with the first mode being a preheat mode at which the IR heating elements operate at approximately half power density and 25-75% (more preferably 45-55%) voltage and with the second mode being a frit sealing mode at which the IR heating elements operate at a half power density and at 50-100% (more preferably 75-85%) voltage.
In certain example embodiments of this invention, a method of making a VIG unit is provided. A VIG subassembly is provided to a heater, with the VIG subassembly comprising first and second substantially parallel spaced apart glass substrates, a plurality of support pillars between the first and second glass substrates, and a frit material for forming an edge seal therebetween. Infrared (IR) energy is emitted from at least one bulb operating at approximately half power density so as to preheat the VIG subassembly. IR energy is emitted from the at least one bulb operating at approximately half power density and at a pre-selected peak IR wavelength at which the first and second glass substrates have an absorption of less than 30% and at which the frit material has an absorption of greater than 50% (more preferably greater than 70% or 80%) in making the VIG unit.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.